Memory cells and their structural features are submitted to a steady process of diminution in order to reduce the area of the cell array and to achieve an ever-growing storage density. This development is to some degree adverse to the requirements of the complementary transistors forming the addressing logic circuits arranged in the periphery of the memory cell array and usually produced in standard CMOS technology, which renders devices of larger dimensions. It is a heretofore unresolved problem, how memory cells comprising transistor structures on a scale of typically 70 nm, especially charge-trapping memory cells, can be integrated with CMOS devices of much larger dimensions on the same semiconductor substrate by a process which does not deviate significantly from standard manufacturing processes.
Memory devices with charge-trapping layers, especially SONOS memory cells comprising oxide-nitride-oxide layer sequences as storage medium, are usually programmed by channel hot electron injection. U.S. Pat. Nos. 5,768,192 and 6,011,725 disclose charge-trapping memory cells of a special type of so-called NROM cells, which can be used to store bits of information both at the source and at the drain below the respective gate edges. The programmed cell is read in reverse mode to achieve a sufficient two-bit separation. Erasure is performed by hot hole injection.
U.S. Patent Publication 2003/0185055 A1 and a corresponding paper of C. C. Yeh et al. entitled “PHINES: A Novel Low Power Program/Erase, Small Pitch, 2-Bit per Cell Flash Memory,” 2002 IEEE, disclose a non-volatile semiconductor memory cell with electron-trapping erase state, which is operated as flash memory and is able to store two bits. The erasure takes place by Fowler-Nordheim tunneling of electrons from either channel or gate electrode into the storage layer of a conventional charge-trapping layer sequence, for example an ONO layer sequence. In programming this memory, electric holes are injected into the non-conducting charge-trapping layer. Hot hole injection can be induced at source and drain, i.e., at both ends of the channel.